Is it worthwhile to do the entire trip from Christchurch to Greymouth return or is the scenery more spectular to Arthurs Pass from Christchurch or is the scenery better on the Arthurs Pass to Greymouth section

The best scenery is from CHC to Arthurs Pass. From there to Greymouth it is mostly a tunnel, which is not bad in itself.

But what are your plans? Do you want to return to CHC or go to the west coast?

As Jed noted, the best scenery is between CChurch and Arthurs Pass. If you intend to return the same day, then one good option would be to disembark at Arthurs Pass, then have lunch and take a short hike. The return train will pick you up a few hours later to take you back to CChurch.

Thanks for the responses
One other question We are going to have a hire-car so does the road parrallel the train most of the way?Is the road trip equally as spectacular or is the train trip better?

The road mostly takes a different route - you will not see train tracks from the road except for right near Arthurs Pass. From Arthurs Pass down to Greymouth, the twisty switchbacked road is much more scenic, as much of the train ride is in tunnels. From the Pass down to CChurch, the train is more scenic, as it passes through gorges, crossing several spectacular trestles.

If you already have a car and you intend to cross from one coast to the other, then you should simply drive across. As mentioned above though, a good day trip then would be to take a round-trip TranzAlpine trip from between CChurch & Arthurs Pass.

We have decided to do the road trip to Arthur's Pass and back to CHC before heading to Picton and then around to the West coastThanks for all info

Consider stopping in the Pass for sight-seeing or a walk. The Devil's Punchbowl trail is only a mile or two and leads to a nice waterfall.

May I ask why you are going back to CHC, when if you drive to Arthur's Pass you will be partway to Picton? Is it that you plan to go through Blenheim, etc?